These are studies in which the same people are restudied and retested over a long period of time.

This is called longitudinal (sp?) studies. 

(I do not know if I spelled it right, sorry)

The truman doctrine was an american foreign policy whose stated purpose was to counter soviet geopolitical expansion during the cold war. it was announced to congress by president harry s. truman on march 12, 1947, and further developed on july 12, 1948, when he pledged to contain threats to greece and turkey